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 EndUser Electronic Edition
http://www.omniflex.com		 2007-04-26 00:00:00
No.17

 

Omnifacts is published by Omniflex Pty Ltd. This newsletter is designed to inform and educate in the areas of monitoring and control.
This publication may be freely redistributed if copied in its entirety.  Portions of this newsletter may be reprinted with permission
What is a rate-of-change alarm?
Ian Loudon

Rate of change alarms differ from normal level alarms in that they respond to the change in a variable over a period of time, rather than the absolute value of the variable. See how these alarms work in greater detail.

How do rate of change alarms work?

Rate of change alarms differ from normal (level) alarms in that they respond to a specified rate of signal change rather than to the crossing of an absolute threshold in the measured variable.

For example:

a. Normal alarm is set to 850 degrees C.

When t < 850C there is no alarm

When t > 850C an alarm is triggered

b. Rate of change alarm is set to 10 degrees C per minute

When the temperature changes from 820 to 835 degrees C over one minute, the alarm is triggered (because the temperature changed by 15 degrees per minute - greater than the 10 degree per minute alarm setting).

When temperature changes from 240 to 255 degrees over one minute, the alarm is also triggered.

When temperature changes from 930 to 935 degrees over one minute, the alarm is NOT triggered(the rate of change is only 5 degrees per minute).

We can see that these alarms only respond to changes, and ignore the actual measured value.

Rate-of-change Alarm setpoints can be positive (signal going UP) or negative (signal going DOWN). The alarm will only be triggered when the rate of change is greater than the rate set as the alarm threshold in the direction of the setpoint. For example if the rate alarm is set to -10 degrees C per minute, then the measured value must be falling at a rate greater than this for the alarm to be triggered. 

NOTE:
Rate of change alarms will only be present while the measured variable is changing. Once the measured variable stops changing, then the alarm will reset.

The rate of change is normally measured over a given sampling period. The reading is effectively averaged over the period of sampling time so instantaneous (quick) changes will be detected but will be suppressed. The measurement can only indicate what the average change over time was. For instance, if a signal changes by 10 degrees in 10 seconds, then the actual rate of change is 60 degrees per minute, but only for that ten second period.
If the rate-of-change alarm is set to a one minute sampling rate, then the rate will be measured as a 10 degree change between the one minute samples (or 10 degrees per minute). If the alarm was set to 15 degrees per minute, then the rate alarm would not have been triggered.


Clever design accomodates an age old signal interface problem for supervisory and control systems.

Omniflex Current Loop Powered Isolator makes the Signal Interface simple accomodating an age old signal interface problem for supervisory and control systems.

In many applications there is a need to convert the 4-20mA into 1-5Volts for an RTU, PLC or DCS etc. This is normally inconvenient because the precision resistor needs to be sourced, mounted and wired independently without causing loop errors. To overcome this obstacle,the LPI version also includes a precision 250ohm resistor that can be connected into the circuit to convert the output into 1-5V within the module.

Simply use the terminal provided for a prescision 1-5V out or if its 4-20mA the use the other terminal.


Omniterm Limit Alarms and Trip Amplifiers support Rate of Change Alarms

TTT - DIN rail mount, universal input, software settable combined dual alarm/trip relay with retransmit

Use the TTT when you require both dual trip functionality and analogue signal for monitoring purposes from the same input sensor. universal input allows the TTT to accept most dc input signal types including thermocouples, resistance bulbs, millivolts volts and milliamps

TTP - DIN rail mount, universal input, software settable dual alarm/trip relay

Use the TTP to monitor any dc instrumentation signal and output two high or low alarm/trips via form C relay contacts. This instrument features easy software configurability without the need for any calibration required to set most input temperature, voltage and current ranges.

 


Omniterm B Series - The Smallest Range of Signal Conditioners with the Largest Range of Solutions

 

OMNIFLEX has been designing and manufacturing electronic products and systems for the automation and control industry since 1965.

OMNIFLEX offers a wide range of signal conditioning capability in a surprisingly small range of products. This is the direct result of the attention paid to lowering the cost of ownership of your signal conditioning.

At a quick glance, most signal conditioning modules look the same. But if you are going to choose a product range you need to consider these factors:

• Installation Costs

• Spares Stock Holding

• Re-calibration Costs

• Reliability

• Plant Safety

Each of these factors has been taken into account in the design of the OMNITERM ‘B’ series range.

Loop Isolators

The elimination of ground loops in your analogue 4-20mA signal loops is the key to reliable and accurate measurements. There are OMNITERM products can solve these system problems, whether you just need to isolate a transmitter from its power source, or whether you need to split the loop into two for the addition of data acquisition or monitoring.

Signal Transmitters

The latest “sigma/delta” conversion technology combined with full software configurability makes these OMNITERM products the most accurate and versatile in their class.

Inputs:
  • Thermocouples
  • Resistance (RTD’s)
  • milliVolts
  • Frequency
  • dc voltage
  • dc current
  • ac voltage
  • ac current

Outputs:
  • dc Voltage (bipolar)
  • dc Current (bipolar)
  • Pulses
  • Inversion

Trip Amplifiers / Limit Alarms

A range of alarm modules are available from simplelimit alarms with manually adjustable set-points to software configurable units capable of rate-of-change alarming direct from the sensor input.

Special Functions - Computation

These modules are designed to solve those tricky special functions required in certain processes, where the sensors require to be conditioned according to a more complex mathematical formula. Available functions include deviation, addition, subtraction, multiplication, linearization, rate-ofchange, peak, valley, ratio, track-and-hold, etc.

 

Last Month:

Last month we talked about several important topics including:
 

  • What is a rate-of-change alarm?
  • Clever design accomodates an age old signal interface problem for supervisory and control systems.
  • Omniterm Limit Alarms and Trip Amplifiers support Rate of Change Alarms
  • Omniterm B Series - The Smallest Range of Signal Conditioners with the Largest Range of Solutions

If you missed these or other key discussions,
you can find the back issues on the newsletters page of our website:

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This publication may be freely redistributed if copied in its ENTIRETY.  Portions of this newsletter may be reprinted with permission.

(c) Copyright 2003 OMNIFLEX PTY LTD 

  
In this Issue
What is a rate-of-change alarm?
Clever design accomodates an age old signal interface problem for supervisory and control systems.
Omniterm Limit Alarms and Trip Amplifiers support Rate of Change Alarms
Omniterm B Series - The Smallest Range of Signal Conditioners with the Largest Range of Solutions
   
Last Month's Issue
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Omniterm B Series Signal Conditioning

 

SIMPLE

RELIABLE

COST EFFECTIVE

Current Loop Isolators

Current Loop Isolators

 

  • Eliminate ground loops
  • Isolate two-wire transmitters
  • Split current loops into two
  • No field calibration required
  • Lowest volt drop in their class

Signal Conditioners

 

Signal Transmitters

  • Temperature Transmitters
  • Frequency Transmitters
  • Universal Transmitters
  • High reliability for SIL1 safety loops
  • Zero field calibration required

 Trip Amplifiers / Limit Alarms

Trip Amps
  • Monitor alarms in your process
  • Connect direct to the sensor
  • Retransmit 4-20mA
  • High reliability for SIL1 safety loops
  • User configurable in minutes

  High reliability and accuracy for optimum process control

  Decrease your installation time by eliminating on site calibration

  Reduce downtime with fast configuration

  Reduce your spares inventory of signal conditioning modules

Select from our versatile range
for your next application